The present invention relates to an optical head apparatus and, more particularly, to an optical head apparatus which is applied to an information recording/reproducing apparatus for recording/reproducing information in/from a recording medium by using a light beam, and increases the recording density by using a super-resolution technique.
In an information recording/reproducing apparatus for recording/reproducing information in/from, e.g., a disk-like recording medium by using a light beam in accordance with a change in shape, optical characteristics, or magnetic characteristics, an increase in recording density is required to increase the memory capacity. To increase the recording density, the diameter of a light spot formed on the recording medium (to be referred to as an optical disk hereinafter) is effectively reduced.
The light spot is formed on the optical disk by condensing a laser beam by an objective lens (condenser lens). For this reason, the diameter of the light spot is proportional to .lambda./NA and cannot be smaller than this value (diffraction limit value) where .lambda. is the wavelength of the incident laser beam, and NA is the numerical aperture of the objective lens. Therefore, development of semiconductor lasers serving as a light source has been progressed to shorten the wavelength .lambda. of the laser beam for reduction of the diameter of the light spot. In addition, the numerical aperture NA of an objective lens is made as large as possible.
However, there almost reaches a limitation in shortening the wavelength of a semiconductor laser or increasing the NA of an objective lens. Hence, in recent years, a super-resolution technique has been studied as a spot diameter reducing technique. The super-resolution technique is conventionally known as a technique for making the light spot size smaller than the diffraction limit value by decreasing the light intensity at the central portion of a light beam and condensing the light beam by an objective lens ("The Resolving Power of a Coated Objective", by H. Osterberg et al., Journal of the Optical Society of America, Vol. 39, No. 7, pp. 553-557, July 1949).
Various optical head apparatuses have been conventionally proposed in which the super-resolution technique is applied to an optical head apparatus to allow recording/reproduction beyond the diffraction limit value (e.g., Japanese Patent Laid-Open No. 2-12623 or 2-12624). In addition, an optical head apparatus has been conventionally proposed which uses, as a technique for modulating the light intensity of a laser source, a cross-shaped light-shielding plate to obtain the super-resolution effect for reducing the spot size in both the x and y directions (Japanese Patent Laid-Open No. 2-195536).
FIG. 10 shows a conventional optical head apparatus using the super-resolution technique, which is disclosed in Japanese Patent Laid-Open No. 2-195536. The conventional optical head apparatus comprises a semiconductor laser 1, a collimator lens 2, a beam shaping prism 41, a light intensity modulator 42, a beam splitter 8, an objective lens 5, and a reproduction signal detection unit 43. A light spot is formed on the information surface of an optical disk 6.
The operation of the conventional apparatus will be described. A laser beam emitted from the semiconductor laser 1 serving as a light source is collimated by the collimator lens 2. The laser beam is shaped by the beam shaping prism 41 and incident on the light intensity modulator 42. As shown in FIG. 11A, the light intensity modulator 42 has a structure with two light-shielding plates 45 crossing each other near the center of a beam section 44 of the incident laser beam, thereby decreasing the light intensity in a cross shape.
The laser beam extracted from the light intensity modulator 42 is transmitted through the beam splitter 8 and irradiated on the information surface of the optical disk 6 through the objective lens 5, thereby forming a small-diameter light spot. The laser beam reflected by the optical disk 6 is reflected by the beam splitter 8 through the objective lens 5 and incident on the reproduction signal detection unit 43. With this operation, a reproduction signal is reproduced.
In this conventional optical head apparatus, the light intensity is decreased in a cross shape by the light intensity modulator 42, as shown in FIG. 11A, and a light spot (main beam) reduced in both the x and y directions to be smaller than the diffraction limit value can be formed on the information surface of the optical disk 6, as indicated by reference numeral 47 in FIG. 11B. With this arrangement, the recording density can be increased, and at the same time, the track density in the radial direction of the optical disk 6 can also be increased. In FIG. 11B, reference numerals 46 denote side lobe light beams of the light spot, which are formed on the x- and y-axes; and 48, a light spot obtained without decreasing the light intensity.
In the conventional optical head apparatus, the light-shielding plates 45 are arranged in both the x and y directions (in the track line direction and a direction perpendicular to the track line direction) to decrease the light intensity. When the light spot is formed on the optical disk 6, the side lobe light beams 46 are generated to irradiate pits before and behind the pit currently irradiated with the light spot 47 formed by the main beam, or adjacent tracks. For this reason, noise is generated, and, a satisfactory reproduction signal cannot be obtained.